How do you prevent thymoma from reoccurrence?

Thymoma recurrence. The best way to prevent recurrence is surgical resection with negative margins in the first place. If surgical margins are positive or "close" consideration to add radiation therapy is strongly recommended.
You can't. Risk is based upon the stage of tumor resected. For advanced stage, we use radiation, and sometimes chemotherapy. Otherwise, careful follow-up with ct scans is all that we can do.
See below. The key is complete resection; with good r0 resection the role of adjuvant radiation is controversial; decision for radiation and chemotherapy post surgery will need to be discussed by the treating team, factoring in stage grade and extent of resection/residual disease;.